Resonant tank circuit for diathermy apparatus or the like



J. A. BOLTSON Dec. 16, 1952 RESONANT TANK CIRCUIT FOR DIATHERMYAPPARATUS OR THE LIKE Filed April 7, 1949 INVENTOR.

JACOB A. BOLTSON Patented Dec. 16, 1952 RESONAN T TANKCIRCUIT'FORDIATHERM'Y APPARATUS'OR THE LIKE Jacob A. Boltson, Flushing,N. Y. HannahBoltson, administratrix of said Jacob A. Boltson, deceased,assignor to Hannah. Boltson, doing business as The Boltson Company,Flushing,

Application April 7, 1949, Serial No. 86,034

8 Claims, 1

Thepresent invention is related to the art concerning resonanttankcircuits, and particularly those adapted for use in short-wave diathermyapparatus requiring high stability of output frequency.

At, the present. state of the art, it is Well known to. use highfrequency short-Wave oscillations for physiotherapy purposes, to produceand/or surface .heating of the body or members thereof for therapeuticbenefit. Diathcrmy apparatus for these purposes essentially comprises ashort-wave oscillation generator in combinationfwith means for usefullyapplying the generated short wave power to the subject to be treated.During normal use of such apparatus extremely wide variations in theconditions of electrical loading are encountered. This produces greatdifliculty in maintaining substantially constant, frequency within therequirements imposed by the Federal Communications Commission. Furtherdifficulty has been encountered in minimizing spurious frequencies, suchas harmonics,,from the generated output.

According to the present invention these difficult'ies. have beensubstantially minimized and almost entirely overcome by the use of anovel form of resonant tank circuit which provides a large degree ofstability despite the wide variation in loading of the oscillator andwhich is inherently self-shielding to minimize production and radiationof spurious frequencies.

In its preferred form, the tank circuit of the present inventioncomprises a flat conductor ar ranged generally in the form of a spiraland completely enclosed in a metallic enclosure which provides acapacitive effect combining with the inductance of the conductor to formthe resonant circuit.

The present invention thereby provides. an extremely simple and highlyuseful, self-shielded,

resonant circuit element, especially adapted for use in the frequencyrange between 5 and 1G0 meg-acyclesper second. In this frequencyrang-3,.

conventional coil and conductor elements for producing resonance presentdifficulties because of the factors of stray and distributed capacitancewhich become troublesome as the Wave lengthbecomes commensurate with thephysical dimensionsof the circuit elements. On the other hand, sectionsof conventional transmission lines cannot be utilized because, at thefrequencies ini volved', their physical length becomes excessive; forexample, at 30 megacycles a quarter-wave line section becomes 2 meterslong, which is too large for practical use.

. By the present invention, a compact, useful internal,

Ill)

tank. circuit is provided of convenient size and with goodefficiency-whichovercomes these disadvantages and usefully employs thenatural capacitance of the device.

The present invention provides a completely enclosed condenser formed ofa conductive plate suspended within and spaced closely to a conductivehousing, the plate and housing forming the two condenser elements. thepath of the current flow in the device is lengthenedto form thenecessary inductive effect. This. is done preferablyby making theinternal condenser element or plate in the form of a flat spiral, as bycutting a generally spiral slot therein to create a coil providingsufficient inductsince, the inductance being a direct functionof thelength of conductive path over which the current is forced to flow.

Other objects and advantages of the present invention. willbecomeapparent from. the follow ing detailed description, taken in conjunctionwith the appended drawing in which:

Figure 1 shows an exploded perspective crosssectional view of apreferred form of tank circuit according to the invention, and

Figure 2 is a plan view of coil ll of the tank circuit of Figure 1, thesectionv of Figure 1 being taken along line I-| of Figure 2.

Referring'to the drawing, the present tank circuit comprises a fiatgenerally spiral conductive coil ll. spiral coil l I may be formed ofconcentric circular arcuate portions [2, l3, Hi, each formed as a nearlycomplete circle. these arcuate portions are joined by radially extendingconnecting portions IE, IS. Thus, at the center the coil II has asubstantially circular disc portion I! joined to the innermost arcuateportion I2 by the radial converting portion l8. Arcuate portion I2 isconnected to the succeeding arcuate portion l3 by the radial connectingportion 15. Similarly, arcuate portion I3 is con-- nected to nextoutermost arcuate portion M by the radially extending connecting portionIS. The coil 1 l is preferably formed of relatively wide strips ofconductive material arranged in the same plane and only slightly spacedfrom one another. In one example which has been constructed, the widthof each of the arcuate portions l2, l3, M was one inch, while the gap l9spacing-the consecutive arcuate portions was 2; inch. Gap 20 betweencenter disc ll and the innermost arcuate portion 12 may have the samewidth as the remainder of the gap l9, or maybe substantially larger,such as of the order of of an inch in the example given.

To provide resonance.

For convenience of fabrication, this,

Consecutive ones of Coil I I is mounted symmetrically between twoparallel circular plates 22 and 23 which are relatively closely spacedto coil II (in the embodiment indicated above this spacing was {a inch).Connecting the two plates 22, 23 is a cylindrical element 24 which isjoined at either end to plates 22 and 23 to form a completely enclosedhousing of pillbox configuration containing and. surrounding the coil II. It will thus be seen that coil II presents a larger area in closejuxtaposition to each of these plates 22 and 23 and therefore has alarger inherent capacitance. It is this capacitance in combination withthe inherent inductance of the coil which provides the resonance of thetank circuit. Coil II is supported within the enclosure 22, 23, 24 by aplurality of insulating studs 26.

The inner disc portion I1 is conductively connected to the lower plate23 by a preferably rigid conductor 21 which passes through an aperture28 at the center of plate 23, and is then curved back to form a loop 29conductively connected to plate 23 as to 3D. This conductor 21,therefore, acts both as a further support for the coil II, and as outputconnection. The outermost arcuate portion or turn I4 of coil I I isprovided with an upward extension 32 which passes through a suitableaperture 33 in upper plate 22 to form a terminal 34.

In customary use the terminal 34 serves as the high potential terminalof the tank circuit, and the housing 22, 23, 24 serves as the lowpotential or ground terminal. Output is derived from the tank circuit byinductive coupling, by means of coupling loop 35 inductively coupled tothe loop 29. It will be appreciated that this coupling is extremely low,since the coupling occurs adjacent the low potential point 30 of thetank circuit. Especially when used in diathermy equipment, this loosecoupling permits the attaining of proper frequency stability since thereaction of the highly variable load upon the frequency of oscillationis thereby minimized.

The use of the extremely large capacitative effect between the coil I Iand the top and bottom plates 22, 23 is highly important in the presentinvention, since this capacitance forms a very low impedance forharmonics or higher spurious frequency, whereby such harmonics orspurious frequencies are greatly minimized. In addition, theself-enclosing and self-shielding nature of the device and the lowpotential output coupling greatly minimize any radiation of suchharmonics or spurious frequencies. Also, the design of the device issuch as to produce extremely low thermal drift, because of the largeheat-radiating area made available. An embodiment of this tank circuitprovided a drift in frequency of only 12 kilocycles at an operatingfrequency of 27.12 megacycles, after one hour of operation under theworst conditions. therefore combines the desirable features ofnonradiation of harmonics and high stability which are especiallyimportant in short-wave diathermy apparatus.

While the coil II has been illustrated as formed of circular arcs andhaving circular arcuate gaps, it will be understood that this form neednot be strictly adhered to, but is preferred solely for ease offabrication when the coil II is machined from a single sheet ofconductive material. However, coil II need not be restricted in itsmanufacture of this method of fabrication. Where other methods offabrications are used, the gap I9 may be made as a spiral gap and theThis tank circuit conductive portion of the coil II may similarly be ofspiral configuration without departing from the essential spirit of theinvention.

In designing the coil II, it is found that in general the dimensions ofthe coil (that is, its diameter, number of turns, width of turns, andspacing of turns) may be empirically determined to provide a closeapproximation to the desired resonant frequency; thereafter, a fineadjustment to attain exactly the desired frequency of resonance isprovided by loop 29. It will be appreciated that the inductance of loop29 is added to that of coil I I, and is thereby reflected into andaffects the resonance frequency of the tank circuit. By making loop 29larger, for example, its inductance is increased and correspondinglyincreases the inductance of the tank circuit, reducing its resonantfrequency. Conversely a decrease in the area of loop 29 serves toincrease the resonant frequency of the tank circuit. In this way a minoradjustment can be made of the resonant frequency of the tank circuitafter fabrication of the coil II.

In the form of the invention described above, the dielectric materialbetween the coil II and the housing 22, 23, 24 is air. Where desired,other forms of dielectric may be used. Thus, where voltage arc-over maybe an important factor, the housing may be evacuated by using insulatingvacuum seals at the aperture 33, 28. Where increased capacitance may bedesired, mica or other high dielectric constant material may be placedbetween coil I I and plate 22 and/or plate 23, or else the entirehousing may be filled with a desirable dielectric material.

While the tank circuit described above is particularly suitable andadapted for use with short wave diathermy apparatus, especially in the27.12 megacycle frequency range, it will be apparent that its utility isnot restricted merely to such apparatus, but it is useful wherever suchtank circuits are desired. Its relatively massive con struction,containing as it does a large area and amount of conductive material,renders it especially adaptable for uses in the high frequency inductiveheating field, where relatively large amounts of power are handled.

In addition to serving as a tank circuit, it is also highly useful as aradio frequency choke coil of the resonant type. When so used, lead 29would not be connected to plate 23 by loop 29 as shown, but would beconnected directly to any desired circuit as a second terminal.

The present apparatus is also useful as a direct-current shorting andhigh frequency isolating element. For this purpose, for example,terminal 34 may be connected to upper plate 22 and the input terminalthen provided by lead 21 (loop 29 being, of course omitted).Alternatively, terminal 34 may be the input terminal, and lead 21 may bedirectly connected to plate 23, without loop 29 being formed therein.Thus either terminal 34 or 21 may be connected to the housing 22, 23,24, or either or both may be isolated from the housing and connected toexter nal circuit elements.

In place of the coupling loop 29, conductor 21 may be led straightthrough aperture 28, and coupled directly to an external circuit.Alternatively, two conductors may be led from coil II through aperture29, one being conductor 2'! (not here coupled to plate 23) and th otherbeing a similar conductor connected to coil II at some pointintermediate center disc I! and outer terminal 34, according to thedesired degree of coupling to the external circuit.

In some instances, it is desirable to provide two tank circuits, as in atuned-grid oscillator, or in a system whereby two tank circuits areparalleled, only ne being coupled to the load whereby the otherstabilizes the frequency being generated. Such a double tank circuit isreadily provided in the present invention merely by stacking two of theabove described circuits one upon the other. In effect, the housing thenbecomes a single container divided into two parts by a.-

septum extending thereacross, with a spiral element symmetricallyarranged in ach part, and the dual circuit can be easily constructed inthis manner.

The present invention therefore provides a simple but efficient resonantcircuit usefully employing deliberately augmented stray capacitance toresonant with an inductance element, whereby the resultant structure hasconvenient size in th megacycle region, accompanied by selfshielding,reduction of harmonics, and high frequency stability.

It will be understood that the details of the device described above aresusceptible of variation in its specific features, and it is merely anillustrative embodiment of the invention, which is not limited theretobut is as defined in the appended claims.

I claim as my invention:

1. A resonant circuit adapted for use in diathermy apparatus or thelike, comprising a generally spiral plane coil, each of whose turns hasa radial width of a larger order of magnitude than the radial separationbetween successive turns, and means providing substantial capacitance toresonate with said coil, comprising a pair of conductive flat platewalls disposed respectively on either side of said coil in paralleladjacent relation thereto, and conductive means connecting said wallsand enclosing said coil whereby said circuit is rendered non-radiatingand harmonic resonances are suppressed.

2. Apparatus as in claim 1 wherein said flat plate walls are closer tosaid plane coil than th radial width of said coil turns.

3. Apparatus as in claim 1, wherein one of said walls is formed with anaperture adjacent to the outermost end of said coil, said coil having aterminal extension connected to said outermost end and passinginsulatedly through said aperture to the exterior of said housing.

4. Apparatus as in claim 1, wherein one of said walls is formed with anaperture centrally thereof, and opposite the center of said coil, saidcoil having a terminal conductor connected to its center and passinginsulatedly through said aperture.

5. Apparatus as in claim 4, wherein said terminal conductor is bent inthe form of a magnetic loop and has its exterior end connected to saidone wall to form a coupling for said circuit.

6. A resonant circuit for diathermy equipment or the like, comprising agenerally spiral plane coil formed of a plurality of concentric arcuateportions, each only slightly less than a complete circle, each of saidarcuate portions having a radial dimension of an order of magni tudelarger than the spacing between successive arcuate portions, eacharcuate portion having one end connected with one end of its immediatelysurrounding arcuate portion by a radial connecting portion to form agenerally spiral coil having circular arcuate portions, means providingsubstantial capacitance for resonating with said coil, comprising a flatconductive plate parallel to said coil and spaced therefrom by adistance less than said arcuate portion radial dimension, and a pair ofterminals connected respectively to the outermost end and to theinnermost end of said generally spiral coil.

7. A resonant circuit comprising a generally spiral plane coil, meansproviding substantial capacitance for resonating with said coilcomprising a pair of flat conductive plates disposed adjacent to andparallel to said coil on either side thereof and a cylindricalconductive wall joining said conductive plates and surrounding saidcoil, whereby said wall and plates form a conductive housing surroundingsaid coil, one of said plates having an aperture substantially centrallythereof and one of said plates having an aperture adjacent the outeredge thereof, and a pair of coil terminals connected respectivelyadjacent the inner and outer ends of said coil and extending insulatedlythrough respective one of said apertures for connecting said resonantcircuit to other circuit elements.

6. A resonant circuit comprising a generally spiral plane coil and meansproviding substantial capacitance to said coil to resonate therewith,comprising a flat conductive plate disposed parallel and adjacent tosaid coil to form a lumped capacitance therewith, said plate having anaperture centrally thereof, and an output coupling comprising aconductor connected at one end to the inner end of said spiral coil andpassing insulatedly through said central aperture.

JACOB A. BOLTSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,837,678 Ryder Dec. 22, 19312,027,861 Fyler Jan. 14, 1936 2,163,775 Conklin June 27, 1939 2,367,576Harvey et a1 Jan. 16, 1945 2,421,137 Wheeler May 27, 1947 FOREIGNPATENTS Number Country Date 223,706 Great Britain Oct. 30, 1924 297,850Germany May 26, 1917

